As a conventional cold cathode electron emitter, there are known a Spindt type electrode, a carbon nanotube (CNT) electrode, and the like. Application of these electron emitters to the field of field emission display (FED) has been considered. Each of these electron emitters applies a voltage to an acute portion to develop a strong electric field of about 1 GV/m, and emits electrons by a tunneling current.
As an example of an idea of causing such an electron emitter to operate in the air, thereby applying the emitter to a charger or an electrostatic latent image forming apparatus, a method for forming an electrostatic latent image by causing a Spindt cold cathode to operate in the air, emitting electrons into the air, and ionizing gas molecules to generate ions serving as charged particles is disclosed in, for example, Japanese Unexamined Patent Application No. Hei 6-255168 (1994).
In addition, the result of a study of causing a carbon nanotube to operate in the air is reported in “Japan Hardcopy 97 papers”, page 221, the Imaging Society of Japan. Herein, there is suggested a probability of the carbon nanotube as a charger for electrophotography or an electron beam source for forming an electrostatic latent image.
Generally, in an electrophotographic process, a surface of a photoconductor is uniformly charged, the surface of the photoconductor is selectively exposed in correspondence with an image to be formed, an exposed portion of the photoconductor is made conductive to thereby discharge charges, and charges or so-called electrostatic latent images arranged on the surface of the photoconductor in correspondence with the images are formed. Thereafter, the photoconductor is passed through a developing section having a developing sleeve that rotates while carrying charged toners on its surface and that is arranged to oppose the surface of the photoconductor, thereby selectively attaching the toners onto the surface of the photoconductor. A transfer section transfers the attached toners onto a sheet of paper. Thereafter, the photoconductor is passed through a charge elimination section that eliminates charges from the photoconductor by irradiating the photoconductor with light. The photoconductor is further passed through a cleaning section that mechanically eliminates residual toners and paper particles attached onto the surface of the photoconductor, and then charged again for next imaging. The charger that uniformly charges the photoconductor is therefore indispensable to the electrophotographic process.
However, each of these two electron emitters generates the strong electric field in the vicinity of the surface of an electron emitting section as described above. Consequently, the emitted electrons are given strong energy by the electric field, and collide against gas molecules to ionize the gas molecules.
Positive ions generated as a result of ionization of the gas molecules are accelerated in an emitter surface direction by the strong electric field, collide against one another, and cause emitter breakdown by sputtering.
As another conventional example of the cold cathode electron emitter, there are known metal insulator metal (MIM) and metal insulator semiconductor (MIS) electron emitters.
Each of these electron emitters is a surface emitter that accelerates electrons using a strong electric field (internal electric field) generated on an insulating film layer within an emitter, and emits electrons from a flat surface of the emitter. Since the electrons accelerated within the emitter are emitted, it is unnecessary to generate a strong electric field outside the emitter. Accordingly, these emitters are free from the problem of the emitter breakdown by sputtering due to ionization of gas molecules differently from the Spindt and CNT type electrodes.
However, the MIM and MIS cold cathode electron emitters have the following problem. If the MIM or MIS cold cathode electron emitter is caused to operate in the air, then fine particles such as dust are attached onto the surface of the emitter, and the surface is covered with the attached fine particles, thereby shielding electrons and reducing electron emission current.
As still another conventional example of the electron emitter, MIS electron emitters each of which accelerates electrons injected into a porous silicon substrate by an electric field, which passes the accelerated electrons through a surface metal thin film by a tunneling effect, and which emits the electrons into a vacuum space using a quantization size effect of a porous semiconductor (e.g., porous silicon) generated by a semiconductor anodic oxidation treatment are disclosed in, for example, Japanese Unexamined Patent Application No. Hei 8-250766 (1996) and “Materials Research Society Symposium Proceeding” Vol. 638.
Each of these MIS electron emitters emit the electrons accelerated by the strong electric field within the emitter similarly to the above-described MIM and MIS cold cathode electron emitters. It is therefore unnecessary to generate a strong electric field outside the emitter and the emitter is free from the problem of the emitter breakdown by the sputtering due to ionization of gas molecules.
Moreover, the cold cathode electron emitter using the porous silicon semiconductor can be advantageously manufactured by quite a simple and inexpensive fabrication method of anodic oxidation.
However, these conventional cold cathode electron emitters operated in the air have the following problem. Since fine particles such as dust are attached onto the surface of the emitter and the attached fine particles shield electrons, the electron emission current is reduced.
Generally, the surface of the MIM or MIS cold cathode electron emitter that accelerates electrons by the internal electric field generated within the emitter also functions as an upper electrode that generates the electric field within the emitter. Consequently, the surface is made of a metal thin film so that the electrons accelerated by the internal electric field can be emitted into the external space while the electrons tunnel through the metal thin film. If the metal thin film is thinner, it is easier for the accelerated electrons to tunnel through the metal thin film. This can increase a tunneling probability and a quantity of emitted electrons.
A thickness of the metal thin film that functions as both the upper electrode for generating the internal electric field and the thin film electrode for emitting the accelerated electrons is preferably several nanometers to several tens of nanometers (nm). Japanese Unexamined Patent Application No. Hei 8-250766 (1996) discloses that the thickness of the metal thin film is, for example, 15 nm.
If the fine particles such as dust are attached onto the surface of this upper electrode (metal thin film), electrons cannot be emitted. It is therefore necessary to eliminate the attached dust. In order to eliminate the dust, a contact type dust elimination method for sweeping off the dust on the surface of the upper electrode (metal thin film) using a cleaning member is normally used. However, the contact type dust elimination method using such a cleaning member has the following problem. If the metal thin film is not treated carefully, the metal thin film is damaged or, in the worst case, peeled off by friction and stress generated while the dust are being swept off.
As another measures against attachment of dust while the MIM or MIS electron emitter is caused to operate in the air, a method using gas introduction means and a particle filter for preventing fine particles from being attached onto the surface of the emitter is disclosed in, for example, Japanese Unexamined Patent Application No. 2001-313151.
However, because of use an air current, this method has problems of low ion utilization efficiency and complicated mechanism.
As described above, the conventional electron emitters are confronted with the problem of the emitter breakdown by the sputtering if the electrons are accelerated in the external space using the external electric field. In addition, the conventional electron emitters are confronted with the problem of a reduction in electron emission current resulting from the attachment of dust onto the surface of the electrode if the electrons are accelerated using the internal electric field within the emitter. Besides, the cleaning method for wiping off the metal thin film electrode on the surface of the emitter so as to eliminate the dust may possibly damage the metal thin film. Thus, if one of these conventional electron emitters is applied to an electron emission device, e.g., a laser printer or a digital copying machine, used for charging the photoconductor thereof in the air as it is, the problems of sputtering of the upper electrode itself and the attachment of dust onto the upper electrode occur. They disadvantageously make it difficult to use the electron emission device for a long period of time.
Thus, the present invention aims to solve the problem of attachment of dust when an electron emitter is caused to operate at an atmospheric pressure. The present invention also aims to provide an electron emission device, an electron emitter cleaning device, and an electron emitter cleaning method capable of stably performing charging and electrostatic latent image formation.